1. Field of the Invention
This invention relates generally to the use of a near field communication antenna and a touch sensing device, wherein the antenna and the touch sensing device are used in close proximity to each other such that the near field communication antenna can interfere with operation of the touch sensing device.
2. Description of Related Art
The present invention describes the use of a touch sensing device in combination with a near field communication (NFC) antenna. The use of the term “touch sensing device” should be considered as interchangeable with the terms “touchpad”, “touch screen” and “touch sensitive device”. Likewise, the term near field communication antenna should be considered as interchangeable with the terms “contactless card reader”, “RFID reader” and “blue tooth antenna”. Furthermore, the “systems” referred to will include a combination of a touch sensing device and a near field communication antenna, using all of the interchangeable terms.
There are several designs for capacitance sensitive touchpads. It is useful to examine the underlying technology to better understand how any capacitance sensitive touchpad can be modified to work with the present invention.
The CIRQUE® Corporation touchpad is a mutual capacitance-sensing device and an example is illustrated as a block diagram in FIG. 1. In this touchpad 10, a grid of X (12) and Y (14) electrodes and a sense electrode 16 is used to define the touch-sensitive area 18 of the touchpad. Typically, the touchpad 10 is a rectangular grid of approximately 16 by 12 electrodes, or 8 by 6 electrodes when there are space constraints. Interlaced with these X (12) and Y (14) (or row and column) electrodes is a single sense electrode 16. All position measurements are made through the sense electrode 16.
The CIRQUE® Corporation touchpad 10 measures an imbalance in electrical charge on the sense line 16. When no pointing object is on or in proximity to the touchpad 10, the touchpad circuitry 20 is in a balanced state, and there is no charge imbalance on the sense line 16. When a pointing object creates imbalance because of capacitive coupling when the object approaches or touches a touch surface (the sensing area 18 of the touchpad 10), a change in capacitance occurs on the electrodes 12, 14. What is measured is the change in capacitance, but not the absolute capacitance value on the electrodes 12, 14. The touchpad 10 determines the change in capacitance by measuring the amount of charge that must be injected onto the sense line 16 to reestablish or regain balance of charge on the sense line.
The system above is utilized to determine the position of a finger on or in proximity to a touchpad 10 as follows. This example describes row electrodes 12, and is repeated in the same manner for the column electrodes 14. The values obtained from the row and column electrode measurements determine an intersection which is the centroid of the pointing object on or in proximity to the touchpad 10.
In the first step, a first set of row electrodes 12 are driven with a first signal from P, N generator 22, and a different but adjacent second set of row electrodes are driven with a second signal from the P, N generator. The touchpad circuitry 20 obtains a value from the sense line 16 using a mutual capacitance measuring device 26 that indicates which row electrode is closest to the pointing object. However, the touchpad circuitry 20 under the control of some microcontroller 28 cannot yet determine on which side of the row electrode the pointing object is located, nor can the touchpad circuitry 20 determine just how far the pointing object is located away from the electrode. Thus, the system shifts by one electrode the group of electrodes 12 to be driven. In other words, the electrode on one side of the group is added, while the electrode on the opposite side of the group is no longer driven. The new group is then driven by the P, N generator 22 and a second measurement of the sense line 16 is taken.
From these two measurements, it is possible to determine on which side of the row electrode the pointing object is located, and how far away. Using an equation that compares the magnitude of the two signals measured then performs pointing object position determination.
The sensitivity or resolution of the CIRQUE® Corporation touchpad is much higher than the 16 by 12 grid of row and column electrodes implies. The resolution is typically on the order of 960 counts per inch, or greater. The exact resolution is determined by the sensitivity of the components, the spacing between the electrodes 12, 14 on the same rows and columns, and other factors that are not material to the present invention.
The process above is repeated for the Y or column electrodes 14 using a P, N generator 24
Although the CIRQUE® touchpad described above uses a grid of X and Y electrodes 12, 14 and a separate and single sense electrode 16, the sense electrode can actually be the X or Y electrodes 12, 14 by using multiplexing.
One problem with integrating a near field communication antenna and a touch sensing device is interference. For example, the strong magnetic field necessary to power a near field communication antenna that is used as a contactless card reader may create strong eddy currents within electrodes of the touch sensing device, thereby causing operation outside of specifications, and malfunctions or inoperability is the result. Similarly, a near field communication antenna can electrically couple to the electrodes of the touch sensing device. Thus, a near field communication antenna may cause magnetic field inductance and electric field coupling with the touch sensing device.
In a related interference problem, the touchpad creates strong electrostatic fields that are necessary to detect a finger. These strong fields often cause the near field communication antenna to have insufficient signal integrity.
The adverse effects of the both electrostatic field coupling and magnetic field inductance may be a result of 1) the near field communication antenna signal causing non-linear effects due to noise/interference signal levels being large enough to trigger ESD diodes in touch sensing device circuitry, 2) difficulty for the touch sensing device front-end electronics or analog-to-digital converters (ADCs) in tracking the interference also causing non-linear effects and error in measurement, and 3) the amplitude modulation frequency of near field communications is often very close to the touch sensing stimulus frequency, thereby creating in-band ground bounce.
It would be a further advantage to dispose the circuitry of the near field communication antenna and the touch sensing device near enough to each other to prevent eavesdropping or tapping into the signals between them to thereby provide an integrated system that is more secure than existing integrated systems. Furthermore, it would be an advantage to remove the electrical and magnetic interaction between them. Finally, it would also be of benefit to integrate the electronics into a single package to address the very limited space of the touch sensing device and the NEAR FIELD COMMUNICATION antenna and associated routing space typical of today's electronic appliances.